MU-MIMO (Multiple User-Multiple Input Multiple Output), which is a communication technology enabling multiple terminals each having plural antennas to communicate simultaneously with one control station having plural antennas, has been a great enabler for high efficiency data transmission in cellular wireless network. There have been many proposals on how to support multi-user transmission on the same MIMO channel [documents 2˜6].
Basically, in terms of channel state information availability at the transmitter, these proposals can be categorized into two classes, one is called “codebook based”, which don not need full channel information at the transmitter, but only the quantized channel vector (in the form of channel vector index feedback), the other one is called “non-codebook based”, which needs full channel information at the transmitter, by means of possible uplink sounding method, which means each user equipment send a common pilot signal through feedback channel so that the base station can detect the channel. The present invention is directed to codebook based MU-MIMO.
Currently, in 3GPP LTE (3rd Generation Partnership Project, Long Term Evolution), there are two main kinds of proposals for MU-MIMO under the codebook based scheme: unitary precoding (document 3) and non-unitary precoding (document 1). “Unitary” means the codeword in the same codeword matrix are orthogonal; on the other hand, “non-unitary” means that the codeword in the codebook are not orthogonal.
FIG. 1 shows schematically the MU-MIMO precoding scheme of the related art. As shown in FIG. 1, the base station schedules users and determines the data rate based on the CQI (Channel Quality Indictor) and PVI (Precoding Vector Index) feedbacked from the user equipments, then the data for each scheduled user can be channel-coded and modulated, and precoded with some weight vector based on PVI, combined with data for other users, and then transformed by IFFT and added by Cyclic Prefix (CP) in case of OFDM scheme, at last transmitted on each transmitter antenna. Here, the IFFT and CP unit can be omitted in case of multiplexing schemes other than OFDM.
In FIG. 1, each user equipment (mobile station) is shown to have a single receiver antenna, however, the user equipments can have plural receiver antennas. The data received by the receiver antenna undergoes CP removal and FFT transform, then user-specific data is extracted by receive combination method (or MIMO detection method). It should be noted that the CP removal and FFT transform units can be omitted in case of any system other than OFDM. At the same time, channel estimation is performed based on common pilot or dedicated pilot, then CQI is computed and PVI is determined before feedback to base station for the next schedule slot.
FIG. 2 shows an example of precoding scheme for 2-user 2-Tx MU-MIMO. As shown in FIG. 2, the data for user 1 (d1) and the data for user 2 (d2) are weighted by vectors [w11, w12], and [w21, w22], respectively, and are added together on each transmitter. In this example, precoding vectors [w11, w12], and [w21, w22] are selected from one common codebook known to both base station and user equipments. At each receiver, the data can be extracted by utilizing the interference avoidance nature of precoding codebook.
In unitary precoding, the codebook with orthogonal vectors can be constructed by some basic math rule, for example, the top nT rows of DFT matrix with the size N (=2B) can be such kind of codebook, as indicated by the following equation,
                                                        f              n                        ⁡                          (              l              )                                =                      exp            (                          -                                                j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                  nl                                N                                      )                          ,                  l          =          0                ,        …        ⁢                                  ,                              n            T                    ;                      n            =            0                          ,        …        ⁢                                  ,                  N          -          1                                    (        1        )            wherein, fn(l) is the l-th element of the n-th vector, nT is the number of transmitting antennas, and N is the size of the codebook, j is the imaginary number. In unitary precoding, the codebook is unitary matrix-based, i.e., N vectors compose P=N/M unitary matrices, where M is the number of transmitting streams, and the p-th unitary matrix is denoted as Fp=[fp, fp+P, fp+2P, . . . ] (p=0, . . . , P−1). The same unitary matrix-based codebook is utilized at both the Node B (base station) and UE side in unitary precoding. In unitary precoding, the CQI can be computed as:
                              CQI          k                =                                            arg              ⁢                                                          ⁢              max                                      i              ,                              j                ∈                                  [                                      1                    ,                                                                                  ⁢                                          …                      ⁢                                                                                          ⁢                      P                                                        ]                                                              ⁢                      (                                                                                                                        H                      k                                        ⁢                                          F                      i                                                                                        2                                                              σ                  2                                +                                                      ∑                                          j                      ≠                      i                                                        ⁢                                                                                                                                    H                          k                                                ⁢                                                  F                          j                                                                                                            2                                                                        )                                              (        2        )            wherein H is a channel matrix, F is a weighting matrix, σ2 is a noise power, and k is an user index.
Note that the CQI computation takes into account all interference from other precoding vector except its own signal. In this case, the CQI is heavily underestimated, so that the throughput of the system is not exploited sufficiently. On the other hand, in non-unitary precoding, the CQI is computed as:
                              CQI          k                =                                            arg              ⁢                                                          ⁢              max                                      i              ,                              j                ∈                                  [                                      1                    ,                                                                                  ⁢                                          …                      ⁢                                                                                          ⁢                      P                                                        ]                                            ,                                                                                                                                      F                        i                                            ⁢                                              F                        j                                                                                                  2                                <                                  ρ                  thrd                                                              ⁢                      (                                                                                                                        H                      k                                        ⁢                                          F                      i                                                                                        2                                                              σ                  2                                +                                                                                                                        H                        k                                            ⁢                                              F                        j                                                                                                  2                                                      )                                              (        3        )            
Here, F is a weighting matrix from a non-orthogonal codebook. Although the CQI computation considers the interference from other streams, but it cannot be guaranteed the user that the BS selects will really use the precoding index determined in the CQI computation. Therefore, the CQI computation will also possibly mismatch with the realistic capacity.
Thus it can be seen that the related art MU-MIMO precoding schemes compute the best CQI, and feedback the CQI and corresponding precoding vector index to the base station. All these schemes without exception either overestimate the interference for unitary precoding, or cause possible mismatch when blindly selecting users for non-unitary precoding. Therefore the throughput of the system can not be maximized.    Document 1: Part 16: Air Interface for Fixed Broadband Wireless Access Systems, IEEE P802.16 (Draft March 2007), Revision of IEEE Std 802.16-2004, as amended by IEEE Std 802.16f-2005 and IEEE 802.16e-2005.    Document 2: 3GPP R1-072422, NTT DoCoMo, “Investigation on precoding scheme for MU-MIMO in E-UTRA downlink”.    Document 3: 3GPP, R1-060335, Samsung, “Downlink MIMO for EUTRA”.    Document 4: 3GPP, R1-060495, Huawei, “Precoded MIMO concept with system simulation results in macrocells”.    Document 5: 3GPP, R1-062483, Philips, “Comparison between MU-MIMO codebook-based channel reporting techniques for LTE downlink”.    Document 6: 3GPP, R1-071510, Freescale Semicoductor Inc, “Details of zero-forcing MU-MIMO for DL EUTRA”.